How much rain before South Florida floods?

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Ken Kaye, Staff writer

After a stormy summer and a soggy September, South Florida is no stranger to ponds in streets and swales. But how much rain does it take to create serious flooding, when water seeps into homes and crests over canals?

To a large extent, total rainfall is not as important as how fast it comes down, weather officials and water managers say. Other variables include how saturated the ground is, how elevated it is and whether an area has good drainage.

In general, two or three inches of rain per hour creates only minor street flooding. Yet if eight to 10 inches pours down in a hour or two, roads usually are severely flooded and canals overflow. That's when the National Weather Service issues flash flood warnings.

"When street flooding begins to close roads and cause stalled vehicles, then we start to get concerned about flooding entering structures," said meteorologist Robert Molleda, of the National Weather Service.

That most recently happened in early June, when the fringes of Tropical Storm Andrea dumped more than 15 inches in Miami-Dade County and up to 10 inches in eastern Broward County, submerging cars and flooding some homes and buildings.

Most of South Florida already is saturated, after one of the wettest summers on record, with much of the region receiving 8 inches more rain than normal. Although a tropical disturbance in the Gulf initially threatened to bring heavy rain this weekend, the weather service now expects it will generate only some scattered showers and storms next week.

Elevation also plays a big role, said Randy Smith, spokesman for the South Florida Water Management District.

"What you'll see in areas that historically have flooding is they're low-lying, even if they're away from the coast," he said.

The South Florida Water Management's flood-control canals in Miami-Dade, Broward and Palm Beach counties generally are designed to handle up to 6 inches of rain with no problems. But if 8 inches fall, water levels will rise to the rim; at 10 inches they likely will overflow, Smith said.

He said there are "too many variables" to determine how much rain would flood the thousands of canals along South Florida's subdivisions. "It depends on when they were built and how they were designed," he said.

Lake Okeechobee hasn't been flooded for several decades, but there have been some major storms that created waves as high as 21 feet. "It's like water in a bowl, rocking back and forth," Smith said.

Although the region has never seen the intense flooding that devastated Boulder, Colo., in the past two weeks, South Florida has been waterlogged numerous times – and tropical systems were frequently to blame.

In August 2012, the fringes of Hurricane Isaac dumped up to 17 inches in western Palm Beach County, turning horse farms into lakes in Wellington and seriously flooding Lion Country Safari.

In October 2000, the so-called "no-name storm" dumped up to 20 inches, mainly over Miami-Dade County. And a year earlier, in October 1999, Hurricane Irene also produced up to 20 inches across much of the region, causing widespread flooding, overflowing lakes and swimming pools and flooding living rooms.

Flooding rains have at least one positive aspect; they make it unlikely a wildfire will break out, said Jeff Radakovic, a ranger with the Florida Forestry Division. "Sometimes lightning will branch off from a storm; a lot fires get started that way," he said.

But downpours also tend to nurture clouds of mosquitoes, which love to lay their eggs in pools of standing water, said Gary Goode, environmental analyst with the Palm Beach County Mosquito Control Division.

"The eggs will hatch and the mosquitoes will start emerging," he said. "That's when we figure we'll have to order an air strike on them."